A pacemaker is a medical device, typically implanted within a patient, which recognizes various dysrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the dysrhythmias. An ICD is a device, also implantable into a patient, which additionally recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate fibrillation.
Pacemakers and ICD's carefully monitor characteristics of the heart such as the heart rate to detect dysrhythmias, discriminate among different types of dysrhythmias, identify appropriate therapy, and determine when to administer the therapy. The heart rate is tracked by the device by examining electrical signals that are manifest concurrent with the contraction and expansion of the chambers of the heart. The contraction of atrial muscle tissue is manifest by the generation of a P-wave. The contraction of ventricular muscle tissue is manifest by the generation of an R-wave (sometimes referred to as the “QRS complex”). Expansion of the ventricular tissue is manifest as a T-wave. Expansion of the atrial tissue usually does not result in a detectable signal. The sequence of electrical events that represent P-waves, followed by R-waves (or QRS complexes), followed by T-waves are sensed using sensing leads implanted inside the heart, e.g., sensing leads.
One commonly used type of sensing lead is the unipolar lead, which includes a single electrode at its tip. The device detects electrical voltage differentials between the electrode and the external body of the device itself. Typically, one unipolar lead is inserted within the atria and another within the ventricles, from which the device derives separate atrial and ventricular channel cardiac signals. Another commonly employed type of sensing lead is the bipolar lead wherein the lead includes two electrodes mounted near its tip. The device detects electrical voltage differentials between the two electrodes. Again, typically, one lead is inserted within the atria and another within the ventricles, from which the device derives separate atrial and ventricular channels of cardiac signals.
A common problem with unipolar leads is that, because the device is sensing voltage differentials between the tip of the lead and the body of the device, significant far-field electrical signals are detected along with the intended atrial or ventricular cardiac signals. A “far-field” signal is a signal originating far from the sensor of the sensing lead, but detected by the sensing lead nonetheless. For example, the atrial cardiac signal derived from the atrial lead will typically include significant ventricular signals. A significant advantage of the bipolar lead is that, because electrical voltage differentials are detected only between two electrodes located closely adjacent to one another at the end of the lead, far-field sensing is significantly reduced. However, bipolar leads are more expensive and are generally perceived as being less reliable than unipolar leads and hence are not preferred by all physicians.
In an attempt to provide the advantages of bipolar sensing using unipolar leads, some state-of-the-art devices employ combipolar sensing techniques. With combipolar sensing, a pair of unipolar leads are mounted within the heart, one in the atria and one in the ventricles. A ventricular channel cardiac signal is generated in the same manner as with conventional unipolar sensing wherein electrical voltage differentials are detected between the tip of the ventricular lead and the body of the device. However, the atrial channel of the cardiac signal is generated by detecting voltage differentials between the electrodes at the tips of the atrial and ventricular leads. For a more complete description of combipolar systems, see U.S. Pat. No. 5,522,855 (Hognelid), incorporated herein by reference.
With combipolar sensing, because the atrial channel is derived based upon voltage differentials between the tips of the two unipolar leads, improved detection of atrial signals is achieved as compared with systems which require the relatively weak atrial electrical signals to be detected based upon voltage differentials generated between the tip of the atrial lead and the body of the device. Ventricular electrical signals are typically much greater in magnitude than atrial signals, hence, with the combipolar sensing technique, it is sufficient to sense the ventricular signals based upon voltage differentials generated between the tip of the ventricular lead and the body of the device. Hence, an overall improvement in the sensitivity of the detection of atrial signals is achieved using combipolar sensing, yet the perceived benefits of unipolar leads are retained, namely that the leads are less expensive and more reliable.
Thus, combipolar sensing provides many advantages. One disadvantage, however, is that, because the atrial channel is detected based upon voltage differentials between the tips of the atrial and ventricular leads, ventricular signals are sensed as “near-field” signals. As a result, ventricular signals may have a greater magnitude on the atrial channel than the atrial signals. Hence it may be difficult to filter the ventricular signals from the atrial channel. (The ventricular channel, because it is detected based upon voltage differentials between the tip of the ventricular lead and the body of the device, may also pick up far-field atrial signals, but these are typically very weak as compared to the ventricular signals and hence can easily be filtered out.)
Regardless of the electrode configuration being used, there is a need for the implanted device to be able to readily and reliably distinguish between various electrical events such as P-waves, R-waves and T-waves. For example, it is of critical importance that the device be capable of recognizing the occurrence of certain atrial arrhythmias based on the sensed atrial rate, and in determining such rate it is critically important that neither R-waves nor T-waves be falsely sensed as a P-wave. Such may be particularly problematic when an combipolar electrode configuration is being used because, as noted, P-waves, R-waves, and T-waves may be sensed as being of the same order of magnitude on the atrial channel. This problem exacerbated during an automatic mode switch (AMS), e.g., when switching the device from a DDD mode to a VVI or DDI mode. DDD, VVI and DDI are standard device codes which identify the mode of operation of the device. DDD indicates a device which senses and paces in both the atria and the ventricles and is capable of both triggering and inhibiting functions based upon sensed events. VVI indicates that the device is capable of pacing and sensing only within the ventricle and is only capable of inhibiting the functions based upon sensed events. DDI is identical to DDD except that the device is only capable of inhibiting functions based upon sensed events, rather than triggering functions. Numerous other device modes of operation are possible, each represented by standard abbreviations of this type.
One technique commonly employed for processing the atrial or ventricular channel signals to eliminate unwanted signals uses “blanking intervals”. With a blanking interval, the device does not process electrical signals during a predetermined interval of time either for all device functions (absolute blanking) or for selected device functions (relative blanking). As one example of absolute blanking, upon detection of an R-wave on the ventricular channel, the device will not detect any signals on the atrial channel during a post ventricular atrial blanking (PVAB) interval. The atrial blanking interval is provided to prevent the device from erroneously responding to a far-field R-wave on the atrial channel. As one example of relative blanking, upon detection of an R-wave on the ventricular channel, the device will ignore all signals detected on the atrial channel during a post-ventricular atrial refractory period (PVARP) as far as the triggering or inhibiting of pacing functions is concerned, but not for other functions such as detecting and recording diagnostic information, particularly detection of premature atrial contractions (PACs). Pacemakers and ICDs may employ both the PVAB and the PVARP, with the PVAB being much shorter than the PVARP interval.
The effect PVAB and PVARP intervals is illustrated in FIG. 1 which shows a stylized representation of one atrial and one ventricular cardiac channel of a normal sinus rhythm detected using combipolar sensing. (Actual devices typically employ multiple atrial and ventricular channels to track different types of information. For example, one atrial channel may be employed for bradycardia detection, whereas another is employed for controlling AMS operations. For clarity in describing the effect of the PVAB and PVARP intervals, FIG. 1 illustrates only a single atrial channel and a single ventricular channel). The ventricular channel includes R-waves and T-waves. The atrial channel includes P-waves as well as ventricular T-waves, detected as near-field waves. FIG. 1 also illustrates the PVAB and PVARP intervals applied to the atrial channel. The PVAB interval, which begins upon detection of an R-wave on the ventricular channel, is set to a duration sufficient to cover the R-wave such that the R-wave is not detected in the atrial channel. The PVARP blanking interval is set to a length such that the T-wave, although detected on the atrial channel, is ignored. Hence, within the atrial channel, for the purposes of atrial rate detection, only events detected outside of the PVARP interval are used for the rate calculation. With proper setting of the PVARP and PVAB intervals, only P-waves are typically detected, and hence an accurate calculation of the true atrial rate is achieved for normal sinus rhythm.
Hence, blanking schemes may be used to blank T-waves from the atrial channel to prevent such T-waves from being falsely sensed as P-waves. However, such blanking schemes have proven less than satisfactory because P-waves may occur during the blanking intervals. Hence, the device may significantly underestimate the true atrial rate, and thereby fail to detect the tachyarrhythmia, flutter or fibrillation occurring in the atria. Thus, improper therapy may be administered.
FIG. 2 illustrates stylized atrial and ventricular channel cardiac signals detected using combipolar sensing during an episode of atrial tachyarrhythmia. As can be seen from the atrial cardiac signal, the frequency of P-waves is far greater than in FIG. 1. As a result, at least two P-waves occur during the PVARP interval, which are therefore ignored for the purposes of atrial rate detection. As a result, the atrial rate is substantially underestimated, possibly resulting in failure of the implantable device to administer appropriate anti-tachycardia pacing. FIG. 2 illustrates an extreme situation wherein a significant underestimation of the atrial rate can occur. But even in more benign circumstances, occasional P-waves may be blanked by the PVARP interval resulting in a slight underestimation of the atrial rate, perhaps sufficient to trigger an unnecessary mode switching or the like.
One solution that has been proposed for providing a better estimate of the atrial rate is to include events detected during the PVARP for the purposes of atrial rate calculations. Using this technique, P-waves detected during the PVARP are thereby sensed. However, T-waves may also be sensed during the PVARP interval resulting in an overestimation of the atrial rate, perhaps sufficient to trigger unnecessary anti-tachyarrhythmia therapy or at least sufficient to trigger an unnecessary mode switch.
Hence, there is a significant need to provide an improved technique for determining the true atrial rate. This need is particularly significant when using combipolar sensing, but also arises when using other electrode configurations such as conventional unipolar or bipolar sensing where T-waves may be detected on an atrial channel as far-field events.